By Ezra Klein

Tuesday's Finance Committee vote is considered such a sure thing that most Hill staffers are already looking past it. When you hear about compromises like an opt-out public option, you're not hearing about a potential amendment to the Finance Committee's bill. You're hearing about a potential addition to the "floor bill." That's where the action is now.

The floor bill will not be the Finance Bill, exactly. And it will not be the HELP Committee's bill. Instead, it will be a blend of the two. The merger will be overseen by Harry Reid's office, and Hill sources expect the room to be fairly small beyond that: Max Baucus, officially representing the Finance Committee, but unofficially representing Senate moderates. Chris Dodd, representing the HELP Committee, but unofficially representing Senate liberals. A few White House staffers (Peter Orszag's name comes up frequently). Finally, Tom Harkin, the actual chairman of the HELP Committee, will have some involvement but isn't expected to be a key player.

Other members of the Senate will have less official roles. The rest of the Senate Democratic Leadership team, for instance, Olympia Snowe. Key liberals and moderates with whom Dodd and Baucus will be in constant communication. But the room itself is expected to be quite small.

The base bill is going to be the Senate Finance Committee's bill. That bill, unlike the HELP Committee's bill, is complete. It has revenues and Medicare and Medicaid. The HELP bill, due to the committee's constrained jurisdiction, lacks all those items. Moreover, everyone wants to keep the Finance Committee's CBO score, or at least something like it. Calling this a merger, then, isn't really accurate. It's more like a bigger company buying out a small rival. The question is what they keep.

The main items under discussion are the public plan and affordability. To secure the support of liberal members, there will need to be some sort of public plan. To retain the support of moderates, the plan will have to continue to bend the spending curve in both the first and second decades. To secure the support of the Democratic caucus more broadly, there will need to be more subsidies, or some other measure to make insurance a bit more affordable. That, however, will require more revenues. No one quite seems to know where those revenues will come from. In fact, the main revenue change under discussion is to soften the excise tax, which would mean the bill raises less, rather than more, money. So that's going to be tricky.

What emerges from this process is expected to look a lot like the final compromise. As a smart Hill source explained to me, the final legislation is hard to change. It won't take anything but the agreement of the members in that room to add something to the bill. But after the bill leaves that room and goes to the floor, it will be very different to make any changes. Harder even than in the committee process, where a bare majority of committee members could add an amendment.

On the floor, you'll need 60 votes to add anything. You also need 60 votes to remove anything. If some group of senators dislikes one of the features, they'll need to beat a filibuster to reform it. Indeed, that means Republicans might not be the only ones employing the filibuster. Eleven moderates and all the Republicans could partner to weaken an element of the bill and still be foiled by 40-some liberals.

The flip side of that, of course, is that you need the entire Democratic caucus to break the final filibuster on the bill. You can't have anyone angry enough that they're willing to scuttle the final legislation. That means the compromise coming out of the merger has to be basically acceptable to all Democrats, and then a core of 41 Democrats needs to be ready to defend every major element from attack (even as some Democrats vote for those attacks). It's not going to be easy. But that's the next step.